Secondary batteries are often provided to users in the form of rechargeable battery packs which can be repeatedly used. In particular, in order to protect users and electronic appliances, lithium ion secondary batteries having a high volumetric energy density typically include several protective circuits incorporated in battery packs for over-charging protection and over-discharging protection to interrupt the output of the battery pack under predetermined conditions.
Some of these protective elements use an FET switch incorporated in a battery pack to turn ON/OFF the output, for over-charging protection or over-discharging protection of the battery pack. However, even in the cases of the FET switch being short-circuited and damaged for some reason, a large current momentarily flows caused by a surge such as a lightning surge, and an abnormally decreased output voltage or an excessively high voltage occurs in an aged battery cell, the battery pack or the electronic appliance should prevent accidents including fire, among others. For this reason, a protective element is used having a fuse which interrupts a current path in accordance with an external signal so as to safely interrupt the output of the battery cell under these possible abnormalities.
PLT 1 discloses a protective element of a protective circuit for a lithium ion secondary battery in which a meltable conductor is connected as a part of a current path from a first electrode, through a conductive layer connected to a heat-generating element, to a second electrode, and this meltable conductor in the current path is blown by self-heating due to an overcurrent or by a heat-generating element provided in the protective element. In this protective element, the current path is interrupted by gathering of the fused meltable conductor in liquid phase onto the conducting layer connected to the heat-generating element.
Furthermore, there has been proposed an LED illumination apparatus in which short-circuit elements are connected to serially connected LED elements in parallel, the short-circuit elements are short-circuited by a predetermined abnormal voltage and normal LEDs emit light (PLT 2). In the short-circuit element of PLT 2, several elements sandwiching an insulating barrier layer having a predetermined film thickness are connected in series.